1. Field of the Invention
The present invention relates to the improvements of a road surface condition determination system for automotive vehicles, and particularly to an anti-skid braking system used on automotive vehicles having a road surface condition determination system.
2. Description of the Prior Art
In recent years, there have been proposed and developed various road surface condition determination systems capable of determining or discriminating the road surface condition by way of processing input information signals from wheel speed sensors provided each road wheel. One such road surface determination system has been disclosed in Japanese Patent Provisional Publication No. 9-020223. The prior art system disclosed in the Japanese Patent Provisional Publication No. 9-020223, has a wheel acceleration arithmetic-calculation section, a filtering section (a second-order high-pass filter), a variance arithmetic-calculation section, and a bad-road determination section. The wheel acceleration arithmetic-calculation section calculates four wheel acceleration/deceleration on the basis of input information signals from wheel speed sensors located at each road wheel. The wheel acceleration/deceleration data will be hereinafter referred to simply as a "wheel acceleration data", since the positive sign of the wheel acceleration data means an accelerating state, whereas the negative sign of the wheel acceleration data means a decelerating state. The filtering section makes the two-order high-pass filtering process with respect to the four wheel acceleration data to produce four high-pass-filtered wheel acceleration data signals for each arithmetic-operation cycle. The variance arithmetic-calculation section calculates a variance DVWB for the high-pass-filtered wheel acceleration data signal (DVWF.sub.(i), DVWF.sub.(2), . . . . , DVWF.sub.(n-l), DVWF.sub.(n)) obtained by a plurality of consecutive arithmetic operations (of a predetermined sampling number of n) executed for each individual high-pass-filtered data signal, from a predetermined expression DVWB={DVWF.sub.(1).sup.2 + . . . +DVWF.sub.(n).sup.2 }/n. The bad-road determination section determines, on the basis of the magnitude of the variance DVWB, as to whether the vehicle is driving on a bad road or on a good road. In arithmetically calculating the variance DVWB within the electronic control unit (ECU), the previously-noted prior art system requires a large number of high-pass-filtered data (DVWF.sub.(1), DVWF.sub.(2), . . . , DVWF.sub.(n-1), DVWF.sub.(n)) obtained by a plurality of consecutive arithmetic operations (of the predetermined sampling number of n) executed cyclically for each individual high-pass-filtered data. The amount of data needed to calculate the variances DVWB.sub.FR, DVWB.sub.FL, DVWB.sub.RR, and DVWB.sub.RL, for the four acceleration data at four road wheels is 4.times.n at the minimum. Such a comparatively long arithmetic-operation time for the data needed to compute the variance DVWB prevents the system from determining or discriminating between bad and good roads in real time. In other words, in the prior art system, there is a phase lag corresponding to a plurality of consecutive arithmetic operations needed to compute the variances (DVWB.sub.FR, DVWB.sub.FL, DVWB.sub.RR, and DVWB.sub.RL). Thus, there is a tendency that the vehicle has already passed through the bad road at the time when the bad-road discrimination section determines that the vehicle is driving on the bad road. Due to such a great amount of necessary data, the system requires increased memory capacities. This increases the total production costs of the system. To provide maximum effective braking depending on the road surface condition such as during bad-road driving on gravel roads or good-road driving on dry pavements, an automotive vehicle with an anti-skidbraking system (ABS) often uses the result of determination of the road surface condition. In the previously-described prior art system, when the ECU determines that the vehicle is driving on the bad road, the ABS operates to set the wheel-brake cylinder pressure at a higher level rather than during the good-road driving, so as to effectively reduce a braking distance of the vehicle. In such an anti-skid braking system which uses the result of discrimination between bad and good roads, in order to effectively extract frequency components corresponding to wheel-speed fluctuations based on input (input vibrations transmitted via the road wheels) from the road surface while removing or eliminating frequency components corresponding to wheel-speed fluctuations occurring owing to skid control from the wheel acceleration data, a higher-order filtering process (containing a second-order filtering process) is often executed. Commonly, such a higher-order filtering process uses a specified cutoff frequency higher than a resonance frequency of unsprung mass in a vehicle suspension vibrating system. Therefore, the road surface condition determination system containing a higher-order filter tends to undesiredly attenuate the amplitude of the frequency component corresponding to the wheel-speed fluctuations (the time rate of change of the wheel speed data). This lowers the accuracy of arithmetic operations, thus deteriorating the accuracy of determination between bad and good roads. The lowered bad-road determination accuracy may result in a lag in the pressure-reduction timing during skid control.